Link adaptation is a component of channel-dependent scheduling in Long Term Evolution (LTE) systems. Link adaptation dynamically adjusts a transmitted information data rate (modulation and coding rate) to match channel conditions for each user. Current LTE systems, in both downlink and uplink, support three modulation schemes. These include Quadrature Phase-Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (16QAM) and 64 Quadrature Amplitude Modulation (64QAM), corresponding to two, four and six bits per modulation symbol, respectively.
Low-order modulation (e.g., QPSK) is robust in that it tolerates a relatively high level of noise and interference, but provides a relatively low bit rate. In contrast, high-order modulation (e.g., 64QAM) provides a relatively high data rate but is more sensitive to noise and interference. Thus, high-order modulation is typically adopted when the signal to interference plus noise ratio (SINR) is sufficiently high. When a modulation scheme is determined, a coding rate is also chosen. The coding rate also depends on channel conditions. A lower code rate is used for lower SINR and higher code rate for higher SINR.
In LTE, a User Equipment (UE) reports Channel Quality Indication (CQI) to assist an eNB in determining an appropriate Modulation and Coding Scheme (MCS) for downlink transmission. Typically, CQI are derived from UE measurements on downlink reference signals. The reported CQI represents the highest MCS that is supported for a Physical Downlink Shared Channel (PDSCH) transmission with a transport block error rate probability not exceeding ten percent. CQI may be signaled to the eNB by the means of a CQI index which indexes a CQI table, such as Table 1. A 4-bit CQI value corresponds to one of the 16 combinations of modulation scheme and coding rate in the CQI table.
TABLE 1CQI index table for PDSCH transmission (Table7.2.3-1 in 3GPP TS 36.213 V11.2.0)CQIindexmodulationcode rate × 1024efficiency0out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
Generally, the modulation schemes and code rates for CQI entries are determined by sampling with approximately 2 dB spacing in the symbol information curves as shown in FIG. 1, where each sampling point matches a corresponding CQI index in the table.
FIG. 1 is a graph that illustrates example symbol information curves for various modulation schemes. The vertical axis represents the modulation bits per symbol and the horizontal axis represents the SINR. The circles numbered 1-15 represent the sampling intervals for the three modulation schemes. Sampling intervals 1-6 coincide with the QPSK symbol information curve, sampling intervals 7-9 coincide with the 16QAM symbol information curve, and sampling intervals 10-15 coincide with the 64QAM symbol information curve.
For downlink data transmission, an eNB typically selects an MCS depending on predicted downlink channel conditions, taking into account CQI feedback. To optimize link adaptation, signaling mechanisms may communicate information between an eNB and a UE. Knowledge of the MCS adopted for PDSCH transmission is indicated by a five-bit field in the Downlink Control Information (DCI). This MCS field corresponds to an index into an MCS table, such as Table 2.
TABLE 2MCS index table for PDSCH transmission (Table7.1.7.1-1 in 3GPP TS 36.213 V11.2.0)MCSModulationTBSIndexOrderIndexIMCSQmITBS0201212223234245256267278289291049114101241113412144131541416415176151861619617206182161922620236212462225623266242762528626292reserved304316
In Table 2, twenty-nine of the thirty-two combinations are used to signal an MCS. The MCS, together with a number of resource blocks, determines the transmit block size (TBS) on the Downlink Shared Channel (DL-SCH). All TBSs can be described by mapping the TBS index, ITBS, and an allocation bandwidth into the corresponding transport block size (in bits) in the TBS table (i.e. Table 7.1.7.2.1-1 in 3GPP TS 36.213 V11.2.0) with 27 rows and 110 columns (maximum bandwidth of 110 resource blocks).
The remaining three entries in Table 2, entries 29-31, are reserved for Hybrid Automatic Repeat Request (HARQ) retransmission. For a retransmission, the TBS remains the same size as in the original transmission and the associated modulation scheme can be adjusted according to the reserved MCS index.
In LTE scenarios with high SINR, such as small cell environments with terminals close to the serving eNB, providing higher data rate with given transmission bandwidth may be achieved with a higher order modulation that carries more bits of information per modulation symbol. For example, 256 Quadrature Amplitude Modulation (256QAM) transmits 8 bits per modulation symbol, which can improve the peak data rate maximum by thirty-three percent as shown in FIG. 2. 256QAM, however, only provides gains when the SINR is sufficiently high. Additionally, current LTE CQI/MCS/TBS tables do not support 256QAM.